A conventional centrifugal fan used for a ventilating blower and air conditioner is disclosed in Japanese Patent Unexamined Publication No. 2002-168194.
Hereinafter, a description is made for this conventional multiblade fan with reference to FIGS. 15 through 20.
FIG. 15 is a sectional side view of a centrifugal fan and casing in the conventional example. FIG. 16 is a sectional view of its substantial part, illustrating a state of airflow between blades in the large air volume and low static pressure zone. FIG. 17 is its sectional side view, illustrating an effective work area in the large air volume and low static pressure zone. FIG. 18 is a sectional view of its substantial part, illustrating a state of airflow between blades in the small air volume and high static pressure zone. FIG. 19 is its sectional side view, illustrating a state of airflow in the small air volume and high static pressure zone. FIG. 20 is its performance characteristic diagram.
As shown in the figure, The spiral inside of casing 1104 is formed at its one side with bellmouth-like inlet 1101 and has discharge outlet 1103, and orifice 1102 with its internal diameter same as blade internal diameter D101. This casing 1104 is provided therein with ring-like lateral plate 1105, and main plate 1107 having throttle 1106 substantially truncated-cone-shaped, projecting toward this lateral plate 1105. A plurality of blades 1109 are attached so as to be interposed by lateral plate 1105 and main plate 1107. Plural blades 1109 has inlet angle 0101 of blade inlet 1108 and outlet angle 0102 of blade outlet 1109, both constant from lateral plate 1105 through main plate 1107. Main plate 1107, lateral plate 1105, and plural blades 1109 compose multiblade fan 1111. Shaft 1113 of motor 1112 mounted to casing 1104 is connected to main plate 1107.
As a result that multiblade fan 1111 is rotated by motor 1112, inhaled air 1114 passes through inlet 1101 of inlet orifice 1102, to flow into blade inlet 1108. Air that has flown into blade inlet 1108 is pressurized between blades 1110, to flow out through blade outlet 109. The air that has flown out is gradually converted from dynamical pressure to static pressure when further passing through spiral casing 1104, to be discharged to discharge outlet 1103. This centrifugal fan changes a load (static pressure) on multiblade fan 1111 according to the length of a discharge duct connected to discharge outlet 1103, resulting in having various operating points from the large air volume and low static pressure zone to the small air volume and high static pressure zone.
In such a conventional centrifugal fan, as a result that the mainstream flows unevenly through the ventral zone of the blades at an operating point in the large air volume and low static pressure zone, exfoliation tends to occur at the dorsal side of the blades. Consequently, the total pressure efficiency decreases to cause turbulent flow noise. Therefore, improving aerodynamic performance with lower noise is demanded.
Next, a description is made for the conventional example disclosed in Japanese Patent Unexamined Publication No. 2001-271791 with reference to some drawings.
FIG. 30 is a sectional side view of a centrifugal fan and casing in another conventional example. FIGS. 31 and 32 are a sectional view and a top view of their substantial parts, respectively.
Spiral casing 2104 is formed at its one side with bellmouth-like inlet 2101 and has orifice 2102 with the same internal diameter as that of the fan and discharge outlet 2103. Casing 2104 is provided therein with main plate 2105, flat and vertical to the rotation axis; a plurality of blades 2106 disposed above and below main plate 2105; and ring-shaped lateral plate 2107 at both sides of the blades. Main plate 2105 is formed thereon with ventilating hole 2108 for air to circulate from top blade 2106a to bottom blade 2106b. Top blade 2106a has blade inlet angle θ201 and blade outlet angle θ202 different from those of bottom blade 2106b. Shaft 2019 of motor 2110 mounted to casing 2104 is connected to main plate 2105. Main plate 2105, lateral plate 2107, and plural blades 2106 compose the multiblade fan.
As a result that motor 2110 rotates, the inhaled air passes through inlet 2101 of orifice 2102, to flow into the blade inlet. The air that has flown in is pressurized between blades 2106, to flow out through the blade outlet. The air that has flown out is gradually converted from dynamical pressure to static pressure when passing through spiral casing 2104, to be discharged to discharge outlet 2103. This centrifugal fan changes a load (static pressure) on the multiblade fan according to the length of a discharge duct connected to discharge outlet 2103. That is, the centrifugal fan has various operating points from the large air volume and low static pressure zone to the small air volume and high static pressure zone.
In such a conventional centrifugal fan, in order to expand to the lateral plate the range of the mainstream unevenly distributed in the main plate, at an operating point in the large air volume and low static pressure zone, blades with different inlet angles and outlet angles are arranged on the top and bottom of the main plate. In such a makeup, the mainstream flows to the bottom blades through the ventilating hole. That is, when passing through the ventilating hole, the mainstream strikes against the main plate and increases its velocity. Then, the flow velocity increases between bottom blades, causing a blade surface to be exfoliated. Consequently, the total pressure efficiency decreases and noise increases.
Meanwhile, at an operating point in the small flow volume and high static pressure zone, the mainstream shifts from blades at the main plate to those at the lateral plate, and the difference between the outlet angle of the top blade and the discharge angle increases if the outlet angle is constant from the main plate through the lateral plate, causing the exfoliation zone to be expanded near the blade outlet. Accordingly, noise tends to occur, and thus improving aerodynamic performance with lower noise is demanded.